In a spread spectrum communication system, an input data stream of a received signal is modulated with a series of spreading code. For example, in GPS (Global Positioning System) system, C/A code is used to modulate signals. In a receiver receiving a spread spectrum signal, it is necessary to generate a local code replica and correlate the local code replica with the received signal, so as to de-spread the signal.
To acquire and track satellites with sufficient efficiency, resolution and accuracy, fractional chip correlation is utilized. In fractional chip correlation, delayed version(s) of the local code replica are correlated with the received signal to synchronize the local code with the code received by the satellite by determining which of the delayed version(s) and the original version is the one which correlates with the received code the most accurately. In practice, the delayed chip space used in the fractional chip correlation should be ½ chip at most in acquisition mode. In tracking mode, the fractional chip space should be less, ¼ or even ⅛, for example, so as to provide sufficient accuracy, since one chip indicates a range of 300 meters in positioning.
FIG. 1 is a block diagram showing a conventional fractional chip correlation device of a receiver, in which the chip space is ¼ chip. FIG. 2 is a diagram showing the waveform at respective components of the fractional chip correlation device in FIG. 1. To perform the fractional correlation for chip space of ¼ chip, the correlator must operate at a speed that is 4 multiples of the sampling rate fs. A signal received by the receiver is down-converted and digitized into an IF (intermediate frequency) signal, which is sampled at a high sampling rate fs, for example, fs=8 fc (fc is the chipping rate of the spread spectrum system). Then the IF signal is latched by a latch 11. The signal is down-sampling by an integration unit 12. The sampling rate fs of down-sampled signal is reduced to 4 fc. As can be seen in FIG. 2, the samples of the down-sampled signal are not as dense as the input signal. The down-sampled signal is then transmitted to four correlators 140, 142, 144 and 146 to be correlated with a local code replica generated by a local code replica generator 16 and delayed versions of the local code replica delayed by delay units 202, 204 and 206, respectively. Specifically, the correlator 140 correlates the signal with the local code replica without delay; the correlator 142 correlates the signal with the local code replica delayed with ¼ chip from the delay unit 202; the correlator 144 correlates the signal with the local code replica delayed with ½ chip; the correlator 146 correlates the signal with the local code replica delayed with ¾ chip. The waveforms of the various versions of the local code replica are shown in FIG. 2.
In the above example, in order to keep up with the fractional chip code transition, the correlation is executed at a high sampling rate, which is 4 fc is this case. The high operation speed results in heavy power consumption, which is a critical problem to a CDMA receiver.